This invention relates to optical communication equipment and in particular to a tunable filter useful in wavelength division multiplexing and demultiplexing.
In recent years, optical fiber technology for telecommunication has progressed rapidly. While the theoretical transmission capacity of the single mode optical fiber has been recognized in the industry as extremely high from the day such fiber was introduced, much of the capacity has not been utilized. For the increasing demand for bandwidth, such as in the transmission of video images and graphics, much attention has been directed lately toward the maximal utilization of the bandwidth of the single mode fiber. Wavelength division multiplexing (WDM) is one of most capable schemes of maximizing bandwidth utilization of single mode fiber.
In a WDM system, multiple optical signal sources emitting spectral components at different wavelengths, .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.n, are coupled into the same single mode fiber by means of a multiplexer into a composite optical signal. After the composite signal with spectral components of different wavelengths is transmitted through the fiber to a desired destination, the multiple spectral components at their respective multiple wavelengths must then be decoupled by means of a demultiplexer into separate optical channels, each spectral component at its wavelength being carried by a different channel to a detector. While the spectral components are said to have their corresponding wavelengths, it is understood that each of the spectral components may comprise light having a small spread (e.g. 0.01%) of wavelengths about a peak characteristic wavelength value.
The channel spacing of a few nanometers or less of WDM system is presently commercially available. Industry standard groups, such as the International Telecommunication Union has specified center wavelengths and channel spacings for the WDM system. Currently, 32 channel WDM system is on the way of commercialization. At the heart of this dense WDM networks are the optical multiplexer and demultiplexer. Many component technologies, including dielectric thin-film coatings, diffraction gratings, embedded fiber grating and planar waveguide phase arrays, have been used to produce multiplexers and demultiplexers. Thin film technology appears best suited to meet the low loss, high isolation objectives for the demultiplexer. However, the control of the center wavelength accuracy of the bandpass of the thin film filter is practically difficult. It is therefore desirable to provide alternative optical components for use in high-reliability WDM multiplexers and demultiplexers.